The Rise and Fall of the Phlogiston Theory of Fire

Overview

At the beginning of the eighteenth century the Phlogiston theory of fire dominated. By the end of the eighteenth century, however, the Phlogiston theory had been overturned by the new concept of the combustion of oxygen. The overthrow of the Phlogiston theory of fire is often presented as a shining example of the triumph of good science over bad, yet the saga is one of many false starts, false experiments, and false assumptions. Personalities, social and cultural influences, and the new emphasis on experimental analysis and natural causes combined to challenge and replace the Phlogiston theory.

Background

The Greek philosophers considered fire to be one of the basic elements of nature, offering a number of different interpretations. Heraclitus of Ephesus (about 535-475 b.c.) made fire the universal force of creation. Aristotle (384-322 b.c.) called fire one of the great principles of all things. Plato (427-347 b.c.), Aristotle's teacher, suggested that burnable objects contained within them some inflammable principle, a substance that made them burn, but it was Aristotle's ideas that dominated medieval European thought.

Aristotle's fire was part of a four-element system consisting of air, earth, fire, and water. A substance such as wood was made up of a combination of the four elements. When it burned the flame was the element of fire escaping, any vapor was air, any moisture water, and the ash that remained was earth.

The sixteenth century Renaissance rediscovered the works of Plato, as part of a wider intellectual movement of rediscovering the classical past. Plato's notion of a burnable principle within a substance fit well with the alchemical ideas of the period. Plato's concept was modified and alchemists came to regard sulphur, or "some vague spirit of sulphur," as the inflammable principle. Sulphur burned almost completely, therefore sulphur was seen as fire itself, or something closely related to fire. A new system of elements was constructed, with substances explained by a combination of sulphur, mercury and salt. So wood burned because it contained sulphur, gave off flame because it contained mercury, and left ash because it contained salt.

In the mid-seventeenth century the observations, experiments, and philosophy of Johann Joachim Becher (1635-1682) and his pupil Georg Ernst Stahl (1660-1734) led them to suggest a new interpretation of sulphur. They proposed that sulphur was actually made from a combination of sulphuric acid plus a new substance they called phlogiston. Phlogiston (pronounced FLO-jis-ton) was actually the principle of fire, not sulphur, and Stahl suggested that phlogiston was released by all substances when they burned. Hence, as wood burns it releases phlogiston into the air, leaving ash behind. Ash was therefore wood minus phlogiston. Sulphur and materials like charcoal and fat burned well because they contained a great deal of phlogiston.

Impact

The phlogiston theory quickly became popular, and was very robust, explaining a wide variety of phenomena. It explained the rusting of metals. As the metal rusted, it gave off phlogiston into the air, so a metal was a combination of its rust and phlogiston. The breathing of animals could also be explained. As food was 'burned' inside the body, phlogiston was released and expelled out of the body by the lungs. Phlogiston was the "motive power of fire," the foundation of color, the principle of inflammability, indestructible, and an "extremely subtle matter." It could easily be used to explain observed results in experiments. For example, experiments showed that if you burned a stick of wood in a confined space, such as a jar, after a short time the combustion would stop. This was explained by suggesting that air could only contain a certain amount of phlogiston, and once it reached its limit then no more combustion could take place.

The theory of phlogiston was very successful, and was so broad in its scope and acceptance it became one of the first unifying hypotheses of the chemical sciences. However, scientists began to have problems explaining some new experimental results. One reason was that the theory tried to explain too many things. The more the theory was modified by its supporters to explain one particular observed behavior, the more difficulty they had explaining others.

The whole method of inquiry into nature was changing. The reliance on the past was shattered by new discoveries and inventions. Challenges to ancient science occurred at the same time as challenges were presented to traditional religion, economics, social structures, and governments. The eighteenth century was a period of revolutions, including the American Revolution, the French Revolution, and between these a revolution in the chemical sciences.

As the theory of phlogiston developed, the nature and properties of the mysterious substance began to be described in different ways. Whereas Stahl had considered phlogiston as a vague principle, the followers of his theory began to assign physical properties such as weight to phlogiston. At first, this seemed only to strengthen the logic of the theory. When wood burns it leaves a lighter substance, ash, behind. Therefore, the missing weight is the escaped phlogiston. When a metal such as iron rusts, the rust appears lighter, so once again the missing weight was the escaped phlogiston.

However, careful experimenters noticed that while the rust of metals appeared lighter, or at least less dense, than the metal it had come from, in fact the rust weighed more. This resulted in more tinkering with the theory. Some supporters suggested phlogiston had a negative weight, and so when it left a substance it made the result heavier. The phlogiston theory began to become unwieldy and overly complicated. Explanations of its properties began to be contradictory. To explain certain properties, sometimes it had to have no weight, sometimes positive weight, and sometimes negative.

Further problems for the phlogiston theory resulted from new experiments and research conducted into gases. An international group of experimenters began working on gases, exchanging research, and publishing and translating experimental results, each bringing their own perspective and assumptions to the results they observed.

In England during the 1770s Joseph Preistley (1733-1804) was a dedicated supporter of phlogiston, but he was also a careful experimenter. He isolated a new gas by heating the rust of mercury. When heated the rust gave off the new gas, and left behind the metal mercury. This new gas made things burn brighter and longer than normal air. Mice sealed in jars of this new gas could breathe for longer than in normal air. Preistley sought an explanation that would remain consistent with the phlogiston theory, so he speculated that this new gas was particularly good at absorbing phlogiston. Ordinary air, he suggested, already contained some phlogiston, and so could quickly be filled up with more phlogiston, making combustion, rusting, and breathing impossible. This new air, which Priestley called dephlogisticated air, was completely free of phlogiston, so it took much longer to fill up.

In France Antoine Lavoisier (1743-1794) performed similar experiments with the same substances. He got the same results as Priestley, but he was seeking a new explanation of combustion, so he saw his results from a different perspective. Lavoisier suggested that rather than phlogiston being given off when a metal rusted, or a substance burned, a more simple explanation was that Priestley's new gas, which he called oxygen, was being absorbed from the air.

While both theories explained the observed results well Lavoisier's explanation had one major advantage over Priestley's, it gave a mechanism for the gain in weight of rusts. The rust of a metal was the metal combined with oxygen, producing a heavier substance called an oxide. This was a revolutionary approach to the problem, breaking with the previous traditions that stretched back to Plato. While common sense suggested burning or rusting an object results in something escaping, Lavoisier's careful experimental analysis showed that in fact oxygen was being absorbed.

However, Lavoisier could not explain the nature of heat and fire, and was forced to invent a strange new substance, which he called caloric. Caloric had a number of similarities to phlogiston in that it was a principle of fire, just as sulphur and phlogiston had been previously considered.

Further experimental work with other metals, their rusts, and other new gases slowly began to develop a more coherent picture of what occurred during rusting and burning. Another breakthrough came with the realization that water was the combination of the gases hydrogen and oxygen. If you burn hydrogen, it produces water. Lavoisier's theory gained support as more and more experiments gave favorable results.

Lavoisier's main opponent, Priestley, outlived him, but was not able to overturn the trend to the 'new chemistry' of Lavoisier. Priestley's last book, published in 1796, still strongly supported the Phlogiston Theory, but did contain a note of surrender to the prevailing opinions of others. He wrote, "There have been few, if any, revolutions in science so great, so sudden, and so general, as the prevalence of what is now usually termed the new system of chemistry, or that of the Antiphlogistons, over the doctrine of Stahl, which was at one time thought to have been the greatest discovery that had ever been made in the science."

While many historians have characterized Priestly as a stubborn, foolish defender of an outdated theory, the acceptance of Lavoisier's ideas in such a short time is more surprising. Critics rightly pointed out that Lavoisier's theory was incomplete, and could not explain all observed results. However, over time the theory grew stronger and more complete, without losing its simplicity. Some accused him of merely substituting Stahl's phlogiston with his own caloric, a substance at least as mysterious. But caloric was not central to Lavoisier's ideas.

The new theory of combustion had several key points in its favor. It was simple, consistent, did not invoke negative weights or other seemingly arcane concepts, and was based firmly on experimental analysis. There remained a few supporters of phlogiston here and there, but the evidence for Lavoisier's theory kept mounting. However, it was not until the twentieth century that the last legacy of phlogiston, Lavoisier's caloric, was explained away. Heat was revealed to be a form of energy, and the mysterious and mythical ideas of caloric and phlogiston were no longer necessary.

DAVID TULLOCH